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Factors Affecting Orientation of OH-Vectors in Micas

Published online by Cambridge University Press:  02 April 2024

A. S. Bookin
Affiliation:
Geological Institute, Academy of Sciences, Pyzhevsky 7, 109017 Moscow, U.S.S.R.
V. A. Drits
Affiliation:
Geological Institute, Academy of Sciences, Pyzhevsky 7, 109017 Moscow, U.S.S.R.
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Abstract

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The orientation of the OH-vectors in hydroxyl groups of micas of different compositions, polytype modifications, and symmetry were calculated by the method of minimization of electrostatic energy. The orientations are strongly affected by the peculiarities of structures, and even slight deviations from the ideal dioctahedral periodicity can introduce a correction of as much as 10° to the value of the polar angle. Ordering of cations in octahedra lead to twisting of the OH-bond towards the octahedron with lower charge and larger dimension. The latter is less true for cation ordering in tetrahedra. Because of these factors and the dependence of the hydrogen position on the coordinates of heavy atoms, such calculations can give reliable results only if the structure has been refined with high precision. Different polytypes of micas displayed the same orientation of the OH-vectors with respect to the axes of a separate layer, if the stacking sequence did not introduce specific distortions of the 2:1 layer.

Резюме

Резюме

Ориентации ОН-векторов гидроксильных групп слюд разного состава, политипных модификаций и симметрии были рассчитаны методом минимизации электростатической энергии. Как оказалось, они сильно зависят от структурных особенностей слюд и даже слабые отклонения от диоктаэдрической периодичности могут внести поправки до 10° к величине полярного угла. Упорядочение катионов в октаэдрах приводит к повороту ОН-связи в сторону октаэдра с меньшим зарядом и большими размерами. Для упорядочения в тетраэдрах этот эффект меньше. Вследствие этих факторов, а также зависимости положения протона от координат тяжелых атомов, такие расчеты могут давать правильные результаты только для хорошо уточненных структур. Различные поли- типные модификации слюд обнаруживают одинаковую ориентацию ОН-векторов относительно осей отдельного слоя при условии, что последовательность наложения слоев не вносит специфических искажений структуры самих 2:1 слоев.

Resümee

Resümee

Die Orientierungen der OH-Vektoren von Hydroxyl-Gruppen in Glimmern verschiedener Zusammensetzung, polytyper Modifikation und verschiedener Symmetrie wurden durch Minimisierung der elektrostatischen Energie berechnet. Die Orientierungen werden sehr stark durch die Eigenheiten der Struktur beeinflußt, und sogar geringe Abweichungen von der idealen dioktaedrischen Abfolge können zu einer Abweichung von bis zu 10° vom Wert des polaren Winkels führen. Die Anordnung von Kationen in den Oktaedern führt zu einer Verdrehung der OH-Bindung gegen das Oktaeder mit der kleineren Ladung und den größeren Dimensionen. Dies stimmt weniger für Kationen in Tetraederlücken. Wegen dieser Faktoren und der Abhängigkeit der Wasserstoffposition von den Koordinaten von Schwermetallen können derartige Berechnungen nur dann verläßliche Ergebnisse liefern, wenn die Struktur mit großer Genauigkeit verfeinert wurde. Verschiedene Polytype von Glimmern zeigen die gleiche Orientierung der OH-Vektoren im Hinblick auf die Achsen einer einzelnen Schicht, wenn die Stapelfolge nicht zu spezifischen Verdrehungen der 2:1 Schicht führt. [U.W.]

Résumé

Résumé

Les orientations de vecteurs-OH dans des groupes hydroxyles de micas de compositions différentes, les modifications polytypiques, et la symmétrie ont été calculées par la méthode de minimisation de l’énergie électrostatique. Les orientations sont fortement affectées parles particuliarités des structures, et même de légères déviations de la périodicité dioctaèdrale idéale peut introduire une correction de jusqu’à 10° à la valeur de l'angle polaire. Le rangement de cations dans les octaèdres mène à la torsion de la liaison OH vers l'octaèdre avec la charge plus basse et ayant les plus grandes dimensions. Ce dernier fait est moins vrai pour le rangement de cations dans les tétraèdres. A cause de ces facteurs, et de la dépendance de la position hydrogène sur les coordonnées d'atômes lourds, de tels calculs ne peuvent donner des résultats dignes de foi que si la structure a été raffinée avec beaucoup de précision. Des polytypes de micas différents exhibent la même orientation pour le vecteur-OH respectivement aux axes d'une couche séparée, si la séquence d'empilement n'introduit pas de distortions spécifiques dans la couche 2:1. [D.J.]

Type
Research Article
Copyright
Copyright © 1982, The Clay Minerals Society

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